Composite wave guide



June 25, 1957 FIG. [3

FIG. /A

ELE C TROMAGNE T/C WAVE CURRENT DENSITY B 2 m F C m m NNN me mTA UAMW mmfin DR PE K R T M a I H m F CURRENT DENS/TY INC/DENT FIG, 3ELECTROMAGNETIC WAVE REFLECTED OFELECTR/C VECTOR INVENTOR- A. M.CLOGSTON A 7 TOR/VE V ate ireei htas This invention relates toelectrical conductors and has as one of its principal objectives theimprovement of electrical conductors with respect to skin effect. Morespecifically, this invention relates to composite conductors employing amultiplicity of spaced, thin, fiat conducting sheets.

This application is a division of application Serial No. 214,393, filedMarch 7, 1951, which issued as U. S. Patent 2,769,148 on October 30,1956.

Due to the phenomenon known as skin effect, at high frequencies thecurrent distribution through a conductor is not uniform. Consider, forexample, the case of a solid conductor to which are applied waves ofincreasing frequency. At zero and sufliciently low frequencies, thecurrent in the conductor is substantially uniformly distributedthroughout and the resistance of the conductor and hence the conductorloss in the line is at a minimum. With increasing frequency, the currentdistribution changes so that the current density is a maximum at theouter surface of the conductor and decreases into the material at a ratedepending on the frequency and the material. In the example given, thecurrent density may be negligible at the middle of the conductor. Thusthe current density in the conductor is associated with a power lossthat is a function of the distribution of current density across thethickness of the conductor.

It is thus a more particular object of this invention to reduce thepower loss associated with skin effect in electrical conductors and morespecifically, in conductors including a multiplicity of spaced, thin,flat conducting sheets.

Further objects of the invention are to reduce the extent to which thepower loss in such a conductor varies with frequency, and to make suchpower loss and consequently its contribution to the attenuation of atransmission line or other wave-guiding structure made up of suchconductors substantially independent of frequency over a broad band offrequencies from the lowest frequency of interest to the highest. F orexample, in practice such a band might be comparatively narrow oralternatively might be sufficiently wide as to accommodate a pluralityof wide-band television channels.

This invention, in one of its more important aspects, resides in acomposite electrical conductor that is sepa rated, transverse to thedirection of desired wave energy propagation, into a multiplicity ofinsulated, fiat conducting elements or laminations of such number,dimensions and disposition relative to each other and the orientation ofthe electromagnetic Wave as to achieve a more favorable distribution ofcurrent and field within the conducting material. The smallest dimensionof the laminations is in the direction perpendicular to both thedirection of wave propagation and the magnetic vector. A convenientyardstick in referring to the thickness of the metal laminations and ofthe insulating layers is the distance 6 given by where 6 is expressed inmeters, 1 is the frequency in cycles per second, ,1. is the permeabilityof the metal in henries per meter, and a is the conductivity of themetal in mhos per meter. The factor 6 measures the distance in which thecurrent or field penetrating into a slab of the metal many times 6 inthickness will decrease by one neper; if e., their amplitude will becomeequal to 1/e=0.3679 times their amplitude at the surface of the slab.

This factor 6 will be called one skin thickness or one skin depth. Inthe case being considered, it is contemplated that the thickness of eachlamination is many times (for example, 10, or even 1000 times) smallerthan 6 (in general, the thinner the better) and that there will be manylaminations (for example, 10, 50, 100 or more). The insulating layersare also made very thin. It has been found that when the conductor hassuch a laminated structure, a wave propagating along the conductor at avelocity in the neighborhood of a certain critical value will penetratefurther into the conductor (or completely through it) than it wouldpenetrate into a solid conductor of the same material. This results in amore uniform current distribution in the laminated conductor andconsequently lower losses. Another way of looking at this result is tosay that the effective skin depth is much larger in the laminatedconductor than the skin depth 6 for a solid conductor of the samematerial as the laminations. The critical velocity mentioned above isdetermined by the thickness of the metal and insulating laminae, and thedielectric constant of the insulating laminae.

In certain circumstances, the metal laminations need not be continuous,as breaks therein will not cause the conductor to be inoperative at highfrequencies.

The invention is applicable to wave guides, cable pairs, and singlecomposite conductors for any of a great variety of uses-to mention justa few types of conductors wherein the present invention can be applied.

The invention will be more readily understood by referring to thefollowing description taken in connection with the accompanying drawingsforming a part thereof, in which:

Fig. 1A is a schematic representation of an electromagnetic wavepropogating through space in the neighborhood of an electricalconductor;

Fig. 1B is a graph of current density vs. depth (distance away from thesurface) in the conductor of Fig. 1A;

Fig. 2A is a schematic diagram showing respectively the directions ofelectric and magnetic field vectors and the direction of propagation ofan electromagnetic wave near the surface of a composite conductor inaccordance with the invention;

Fig. 2B is a graph having the same coordinates as in Fig. 1B, andshowing the increased skin depth produced by the conductor of Fig. 2 ascompared with that of Fig. 1A;

Fig. 3 is a graphical representation showing an electromagnetic wavebeing reflected from an extended'metal surface laminated in accordancewith the invention;

Fig. 4 is a perspective View of a wave-guide structure in accordancewith the invention.

Referring more particularly to the drawings, consider an electromagneticwave propagating through space in the neighborhood of, and parallel tothe surface of an electrical conductor such as copper, silver oraluminum, for example. This situation is shown diagrammatically inconnection with the conductor 10 in Fig. 1A which can be representativeof many phenomena. It can illustrate, for instance, the transmission ofan electromagnetic wave through a coaxial line, or along an open orshielded two wire system, or a wave propagating through a metal Waveguide. It can also represent the situation in the vicinity of atransmitting or receiving antenna. Clearly a very broad class ofelectrical phenomena involving the transfer or periodic oscillation ofelectromagnetic energy in the vicinity of electrical conductors isrepresented in Fig. .LA.

The wave propagating in Fig. lA'is necessarily accompanied by electriccurrents flowing in the metal. Because of these currents, power isremoved from the electromagnetic field and dissipated in the metal. Thiseffect is nearly always undesirable. The distribution of this current ina direction away from the surface is shown in Fig. 1B where it has beenassumed .that the conductor is thick compared to 6, if the frequency issufficiently high. Because of the well-known skin effect, most of thecurrent flows in a thin layer near the surface. The distance from thesurface at which the current density has fallen to 1/e=0.3679 times itsvalue at the surface is known (as mentioned above) as the skin depth andis denoted by 6. The distance is expressed in terms of the frequency (I)under consideration and the permeability (,LL) and conductivity of themetal in Equation 1 above. Within a given amplitude of theelectromagnetic wave, the amount of power lost to the metal will beproportional to 1/617. Referring to Equation 1, it can be seen that thepower loss is proportional to 1/ V0 so that normally the power loss isminimized by choosing a metal of high conductivity, such as copper orsilver.

Suppose that it were possible to arbitrarily increase 6 without greatlychanging a. It is clear that in such a situation the power loss from theelectromagnetic wave would be greatly decreased. It has been discoveredthat it is possible to do just this thing, and the present invention isbased on this discovery. A simple embodiment of the invention will beconsidered first and then more general cases will be discussed.Referring to Fig. 2A, there is again shown an electromagnetic wavepropagating near the surface of an electrical conductor '20. Therelationship of the electric and magnetic vectors and of the directionof propagation of the electromagnetic wave are shown. The conductor inFig. 2A is no longer a solid piece of metal but is composed of manyspaced laminae 21 of metal of thickness W arranged parallel to thedirection of propagation and parallel to the magnetic vector as shown.These laminae are veryth'in compared to 6 and are separated by emptyspace or any appropriate dielectric 22 such as air, polyethylene,polystyrene, quartz, or polyfoam, for example, the thickness thereofbeing represented by t. Whatever the dielectric is, suppose that itsdielectric constant is 6,, and suppose that the conductivity of themetal is u, as before. Fig. 2A is representative of many situations ofwhich a few will be indicated later. The particular cases beingconsidered in which the magnetic vector is parallel to the surface ofthe composite conductor are not representative of all cases, as will beindicated below.

Since the stack of metal laminae in Fig. 2A will not conduct directcurrent in a direction perpendicular to the plane of the laminae, it ispossible by conventional means to measure an average dielectric constantassociated with this direction. This average dielectric constant will bedenoted by Zand is given by the expression ;=e1(1+W/t) farads per meterhave in a medium of dielectric constant 2 and permeability 1. Thiscondition can be arranged by properly disposing suitable dielectricmaterial in all or part of the region traversed by the wave outside thestack.

Under the conditions mentioned, if W is small compared to 6, we candefine an effective skin depth 6 by If the stack of laminations isseveral times 6 in thickness, the current density will decreaseexponentially into the stack and be reduced by one neper at a distancebelow the surface equal to 6 This increased or effective skin depth isshown in Fig. 23. Furthermore, the effective conductivity Oe of thestack of laminations in the It is immediately observed that the powerlost from the electromagnetic wave has been reduced by a factor Forinstance, if the laminae in a typical case are skin depth thick, thepower taken from the wave will be only ,3 of the power that would belost to a solid conductor.

The increased skin depth described above not only is effective ingreatly reducing conductor losses, but has a further major concomitantadvantage. Referring to Equation 1, it can'be seen that conductor lossesgenerally increase as the square root of the frequency. This variationwith frequency very often is equally as troublesome as the lossesthemselves. A simple but extremely wasteful way to reduce this effect isto make the metal conductor very thin. Suppose for instance that theskin depth is 6 at the highest frequency under consideration. If theconductor is not thicker than 5 the losses will clearlyremain uniform,but high, from very low frequencies up to this maximum. Similarly, withthe arrangement of Fig. 2A the size of the stack can be limited to thethickness 6 determined by Equation 3 at the highest frequency, andthereby obtain uniform loss. But since 6 may be made as large as desiredby making W small enough, this uniform loss can be achieved withoutaccepting greatly increased losses at the lower frequencies. By way ofexample, it is desirable to make each insulatingand conducting laminaless than one thousandth of an inch thick. The general situationindicated in Fig. 2A can have many specific embodiments and variations.

Fig. 3 illustrates an electromagnetic wave being reflected from anextended metal surface 70 laminated as above to have a multiplicity ofalternately positioned metal layers 71 and insulating layers 72 toreduce losses incident upon reflection; One component of this wave maybe considered as traveling parallel to the surface and the-othercomponent perpendicular to the surface. Clearly, the conductor lossesassociated with the component of the wave traveling parallel to thesurface can be reduced below those encountered with a solid sheet ofmetal by use of laminations.

In Fig. 4, there is shown a section of a wave guide 80, a pair of whosewalls 81 and 82 have been covered with thin metal laminations 83separated by insulation 84 as before. The two walls 81 and 82 areconnected by walls 85 and 86 of solid metal. Here also, decreasedattenuation can be realized if the electromagnetic wave propagates downthe guide with a velocity in the neighborhood of that appropriate to theaverage dielectric constant of the stack. Obviously, other wave-guidearrangements utilizing this principle at e possible.

It is obvious that many changes can be made in the embodiments describedabove. The various embodiments and the modifications thereof describedherein are meant to be exemplary only and they do not by any meanscomprise a complete list of conductors to which the present invention isapplicable and it is obvious that many more will occur to those skilledin the art. It is intended to cover all such obvious modifications asclearly fall within the scope of the invention.

What is claimed is:

1. In an electromagnetic wave guiding system, a conductor mediumcomprising a multiplicity of flat elongated conducting portions spacedby means including insulating material, and means for launching highfrequency electromagnetic Waves in said system, there being a suificientnumber of conducting portions to carry a substantial por-. tion of thecurrent induced by said waves, the thickness of said conducting portionstransverse to the direction of wave propagation down the length thereofbeing small compared with its appropriate skin depth at the highestfrequency of operation with said high frequency waves and the spacingbetween at least some of said conducting portions being less than theskin depth, whereby the said conducting medium is substantiallypenetrated by the electric field of said waves.

2. The combination of elements as in claim 1 in which each of saidconducting portions is a thin sheet.

3. The combination of elements as in claim 1 in which each of saidconducting portions is a thin sheet and said sheets are arranged in twogroups, the two groups being separated from one another by a largerdistance than the spacing between any two of the conducting portionswithin a group.

4. The combination of elements as in claim 1 in which each of saidconducting portions is a thin sheet and said sheets are arranged in twogroups, the two groups being separated from one another by a largerdistance than the spacing between any two of the conducting portionswithin a group, and there is dielectric material between the two groups.

5. The combination of elements as in claim 1 in which said conductingportions are arranged in flat stacks form'- ing opposite walls of arectangular wave guide.

6. The combination of elements as in claim 1 wherein each of saidconducting portions has a thickness of less than one-thousandth of aninch.

7. The combination of elements as in claim 1 wherein said flatconducting portions are arranged in a stack of more than ten portionsand more than ten insulators, each of the conductors and insulatorshaving a thickness which is less than one-thousandth of an inch.

References Cited in the file of this patent UNITED STATES PATENTS2,008,286 Leib July 16, 1935 2,231,602 Southworth Feb. 11, 19412,433,181 White Dec. 23, 1947 2,676,309 Armstrong Apr. 20, 1954

